Ultrastructural changes of the microvillar cytoskeleton in the photoreceptor of the crayfish Orconectes limosus related to different adaptation conditions

In the retina of crayfish microvilli of seven of the eight photoreceptor cells build highly organized structures, the rhabdoms. Cytoskeletal elements inside the microvilli were investigated in conventional and slightly extracted electron microscopical preparations. In conventional preparations the ultrastructure of these cytoskeletal elements depended on the adaptational state of the animal. They appeared as central filament-like structures inside each microvillus when dark-adapted retinae were prepared and fixed at night in the absence of calcium. Changes of these conditions (light, daytime, or calcium concentration) impaired the detectability of these central filaments; in light-adapted eyes prepared at midday they were rarely seen. Nevertheless, single microvillar filaments were present in light-adapted retinae after mild cell permeabilization with the saponin beta-escin. They appeared as a regular structure in each microvillus, often attached to the membrane. Their fine structure was consistent with the ultrastructure of single actin filaments as indicated by fast-Fourier-analysis and further supported by the presence of anti-actin immunoreactivity in electron microscopical and immunocytochemical preparations. These results indicate that microvillar filaments are not necessarily destroyed by light as previously described; we suggest that their appearance inside the microvillus might be altered by the properties of associated, maybe sidearm-like proteins.